The sound of a pant grunt (shown in the above video, provided courtesy of lead researcher Ms Marion Laporte)

Chimpanzees are aware of the social impact of their communications, primatologists have discovered.

Chimps communicate using a variety of calls and gestures, including making vocalisations known as pant grunts, which signal subordination.

But researchers have found that chimps will change what they "say" depending on who is listening.

That reveals a previously unrecognised social awareness that has implications for the origin of human language.

Details of the discovery are published in the journal Animal Behaviour.

The surprisingly high awareness of the potential social consequences of calling has not been shown so far in any non-human primate

Professor Klaus ZuberbuhlerUniversity of St Andrews

PhD student Ms Marion Laporte of the University of St Andrews in Fife, UK studied a population of chimps living in the Sonso community of Budongo Forest in Uganda.

The Sonso chimpanzees have been followed by researchers without interruption since 1990.

Together with Professor Klaus Zuberbuhler of the University of St Andrews, Ms Laporte recorded how often, and in what context, the chimps communicated with one another using the pant grunt vocalisation (heard in the above video).

"However, there is considerable acoustic variation within these four call types, which makes them difficult to study," says Prof Zuberbuhler.

"Our research is beginning to show that much of this variation is closely linked with different external events and therefore very meaningful to others."

Chimps specifically use the pant grunt to signal they are subordinate to another ape. A higher ranking group member will never use it in front of a lower ranking member of the group.

"Because of this, they were thought to be rigid expressions of rank relations," says Prof Zuberbuhler.

"Our study shows this is not true. Callers exhibit a high degree of social awareness when producing these signals."

Ms Laporte and Prof Zuberbuhler's research, partly funded by the UK's Edinburgh Zoo, documented all the times when subordinate female chimps did, and more important, did not not make a pant grunt vocalisation when meeting a higher ranked chimp.

Then then analysed the data to understand when and why the chimps communicate this way.

The researchers found that female chimps modify their use of the pant grunt signal depending on which other chimps are listening.

If the alpha male was in earshot, the female chimps often refused to greet another male.

The presence of the alpha female also inhibited lower ranking chimps from communicating.

The researchers do not yet know exactly why the chimps stop calling in certain social situations.

One idea is that they stop deferring to other males in the presence of the alpha male to ensure he does not become aggressive.

A more provocative suggestion, say the researchers, is that females may allocate their pant grunts strategically, to help their favourite males climb the social ladder, though this idea remains speculation.

"Pant grunts are very special vocalisations in the chimpanzee repertoire because they are directed at specific individuals," says Ms Laporte.

"And still, surrounding individuals seem to be taken into account before producing this vocalisation."

Prof Zuberbuhler agrees: "What we found was a surprisingly high awareness of the potential social consequences of calling, something that has not been shown so far in any non-human primate," he told the BBC.

The finding that at least one species of great ape is socially aware in this way may have implications for our understanding of how our own language evolved.

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